1 Puddling. 75 these are chemically the same, and the preliminary refining is very often dispensed with, we will give our attention simply to the preparation of wrought iron by puddling. The object of puddling is to eliminate the graphite entirely, and the combined carbon so far as to leave only about '25 per cent., which actually increases the strength of the iron. In the rolling mill, where the metal is rolled into plates or bars, scales of oxide of iron (Fe2 Ga) are formed by the contact of the hot iron with the air. These scales, being broken off, are collected for the puddling furnaces, their use being that of absorbing the carbon from the iron, exactly in the way already described for malleable cast iron. Being intimately mixed with the brokenf white pig in the puddling furnace, Fig. 85, and subjected to a powerful flame, the O from the oxide unites with the C of the iron, and passes away as C 02 gas. Any silicon that is present in the iron unites at the same time with some 0 and forms Si O2, so that the iron is left comparatively pure. During the process, the iron is in the form of a spongy mass, and absolute contact of it with the scales of oxide, now liquid, is ensured by the introduction of a long rake through a small opening in the side door, for the purpose of stirring the whole well together.* As the puddling nears completion, the metal is kneaded by the rake or paddle * To avoid hard labour and increase the output, there are many mechanical furnaces now in operation, notably Danks' Rotary Furnace and the Pernot Revolving Hearth.